import java.util.Collections;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class Concurrenty {
	public static void main(String[] args) {
		Map map = new HashMap<String,String>();
		map.put(null, null);
		map.put(null, "1");
		System.out.println(map.get(null));
		
		//Hashtable
		Map<String, String> normalMap = new Hashtable<String, String>();
		normalMap.put(null, "hashtable");
		System.out.println(normalMap.get(null));
		
		//synchronizedMap
		Map synchronizedHashMap = Collections.synchronizedMap(new HashMap<String, String>());

		//ConcurrentHashMap
		Map concurrentHashMap = new ConcurrentHashMap<String, String>();
		concurrentHashMap.put(null,"key");
	}
}
